radv: Make FragCoord a sysval

radv: Make FragCoord a sysval

load_fragcoord is already handled in common code for radeonsi, so we
don't need to do anything to handle it. However, there were some passes
creating NIR with the varying, so we switch them over to the sysval. In
the case of nir_lower_input_attachments which is used by both radv and
anv, we add handling for both until intel switches to using a sysval.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>